1. Field of the Invention
This invention relates to a charged-particle beam optical apparatus including a specimen holder which is mounted in at least one support member of the apparatus and vibrates when the support member is subjected to shock.
2. Description of the Prior Art
Charged-particle beam optical apparatus of the foregoing type are known in the art. See, for example, U.S. Pat. No. 4,058,731 which relates to an electron microscope of the foregoing type. The cause of such vibrations are, among other things, interfering soil vibrations which are first transmitted to the housing of the charged-particle beam optical apparatus and from there via the support of the specimen holder to the latter itself. At the support of the specimen holder, these interfering soil vibrations have on the average an amplitude between 1 and 10 .mu.m. The vibrations are transmitted to the housing and the support in a most pronounced manner if the frequency of the vibrations is in the neighborhood of the resonance frequency of the housing support, namely 1 to 10 Hz, and resonance peaking is then obtained. The reason a resolution of a few Anstroems is possible in electron microscopes in spite of a vibration amplitude of the specimen holder of several .mu.m is the friction coupling of the important parts of such microscopes to each other which vibrate in phase and with the same amplitude. Thus, for example, part of the specimen holder is mounted, friction-coupled, in its support which in turn can be mounted, friction-coupled, on a specimen stage. The specimen stage in turn rests, friction-coupled, on the upper pole piece of the objective lens of the particle beam apparatus.
Elongated parts of the specimen holder which are not friction-coupled to adjacent parts, on the other hand, can be excited by the support vibrations to resonance vibrations which lead to a change of position of these parts of the specimen holder relative to the support of the specimen holder. If, for example, the specimen holder consists of a rod which passes through the wall of the electron microscope and is secured there in a first support and the other end of which, situated in the interior of the apparatus, engages a counter-support which is part of an adjustable specimen stage, then the portion of the rod-shaped specimen holder between the two supports can be excited to flexural vibrations in the two directions perpendicular to the rod axis. Additional vibrations can also be excited in the rod direction since at least one support must have spring action in the rod direction for adjusting the specimen holder in this direction without play.
The higher the resolution or the accelerating voltage of the electron microscope, the thicker the required magnetic lenses of the microscope must be. This, however, leads to a longer rod-shaped specimen holder and, thus, to a lowering of its resonance frequency. This resonance frequency thereby approaches the resonance frequency of the microscope column, whereby the vibrations of the latter are again transmitted more strongly to the rod-shaped specimen holder.
For maximum resolution (1 A), the position of the specimen must not change more than 0.1 A. Since the amplitude ratio of the interfering vibration of the specimen holder to the forcing support vibration is inversely proportional to the square of the lowest resonance frequency to the excitation frequency, it has already been attempted to make the amplitude of the interfering specimen holder vibration small by keeping the vibration frequency of the column low compared to the resonance frequency of the specimen holder vibration. For this purpose, a low-frequency microscope support, for example, an elaborate air spring system, is known.